Various wireless communications systems such as WLAN (wireless local area network), WiMAX (registered trademark), Bluetooth (registered trademark), etc. have recently been rapidly spreading, requiring smaller, thinner and lighter wireless communications apparatuses using them. Required in accordance therewith are small antennas for wireless communications apparatuses usable in various frequency bands.
JP 09-162633 A discloses a capacitance-coupled-feeding, surface-mountable antenna as shown in FIG. 32. This antenna 132 comprises a radiation electrode 122, a feeding terminal 127 and a grounded terminal 128 formed on a substantially rectangular parallelepiped substrate 121 made of a dielectric or magnetic material. The radiation electrode 122 extends in a substantially loop shape on upper and side surfaces of the substrate 121, having an L-shaped end portion on the upper surface of the substrate 121. The feeding terminal 127 formed from the side surface to the upper surface of the substrate 121 has an L-shaped end portion on the upper surface, which is capacitance-coupled to the L-shaped end portion of the radiation electrode 122. The grounded terminal 128 is formed on the side surface of the substrate 121, such that it is connected to another end of the radiation electrode 122. A mounting board 131, on which the antenna 132 is disposed, is provided with a feeding electrode 125 and a ground electrode 126. The antenna 132 is mounted on the mounting board 131, such that the feeding terminal 127 is connected to the feeding electrode 125, and that the grounded terminal 128 is connected to the ground electrode 126. The ground electrode 126 is not formed in a region 124 of the mounting board 131, which is covered with the antenna 132.
In the antenna of JP 09-162633 A having a gap 123 on an outer surface of the substrate 121, the opposing length and gap of the L-shaped end portion of the radiation electrode 122 and the L-shaped end portion of the feeding terminal 127 can be changed by trimming, etc., to adjust coupled capacitance, thereby easily changing the impedance. In a casing of a wireless communications apparatus, however, the coupled capacitance is highly affected by nearby elements, so that the mere adjustment of impedance likely fails to provide the antenna with good antenna characteristics and high gain.
Also, a radiation electrode formed on the substrate has a limited length, likely resulting in an insufficient radiation electrode length as the antenna becomes smaller. Signals should be amplified to make up for small gain due to insufficient line length, needing larger power for amplifiers. As a result, batteries contained in wireless apparatuses become larger, failing to make the wireless apparatuses smaller. Further, the antenna of JP 09-162633 A would not be able to handle different frequency bands (for example, different communications systems) if used alone.